Three astronauts already in space for six months are now stranded in orbit after their craft may have been damaged by dangerous debris floating around Earth.

China's Manned Spaceflight Agency (CMSA) has revealed that crew from its Shenzhou 20 mission will need to stay on board the Chinese station Tiangong.

Their replacements had already arrived at the space lab after taking off in their own rocket on October 31.

Astronauts Chen Dong, Chen Zhongrui, and Wang Jie have been on Tiangong, which means Heavenly Palace, since April, completing science experiments, carrying out space walks, and living in zero gravity.

At some point before the trio officially handed over the station to the Shenzhou 21 crew on November 5, the capsule the astronauts were supposed to use to return to Earth was struck by an unknown object in Earth orbit.

The collision left a noticeable mark on the space vehicle's hull, leading the team to fear Shenzhou 20 may have also sustained internal damage that could endanger their flight home.

The two astronaut crews will now have to comb through the spacecraft to check for any critical problems before attempting to use the craft again. The next return data has not yet been released.

It's the second time this year that an astronaut crew has been stranded in space due to a problem with their space capsule, taking place just seven months after NASA's Suni Williams and Butch Wilmore were rescued from the International Space Station.

Chinese officials are unsure what struck the spacecraft, but the damage is believed to have been caused by space debris, which is basically junk floating around the Earth from older space missions and rocket launches.

This could include anything from bits from broken satellites and dropped tools from spacewalks to giant pieces of discarded rocket parts.

This floating garbage is moving around the planet at incredible speeds of up to 17,000mph, the average velocity for objects in low Earth orbit.

This can be a real hazard for manned spacecraft, similar to driving through a hailstorm of bullets.

Right now, there are about 19,000 pieces of space debris in Earth's orbit that the US is tracking, not counting the satellites still working. 

However, experts at NASA think there could be over half a million smaller pieces too small to track easily.

'The impact analysis and risk assessment are underway,' officials at CMSA said, according to CNN.

However, the agency did not provide any details on how long it might take the crews of Shenzhou 20 and 21 to check the ship and repair whatever damage was done.

This isn't the first time space debris has struck a manned spacecraft while astronauts orbited the Earth.

Russia's Mir space station was repeatedly struck by tiny pieces of space junk during its lifespan in the 1980s and 1990s.

The International Space Station (ISS) has also been on the receiving end of several collisions with debris, including some incidents that caused significant damage to the US and Russian-led space lab.

In 2021, a piece of space junk left a hole in one of the station's robotic arms.

Just a year later, an unknown object hit a docked Russian Soyuz spacecraft, putting a nearly inch-wide hole in its radiator and causing a coolant leak.

As for the crew of Shenzhou 20, it's not currently known what the backup plan will be if the damage to their craft cannot be repaired.

The Shenzhou 21 mission was also scheduled to last for six months on board the station, which is exclusive to Chinese astronauts.